1. Field of the Invention
The invention relates to a method for producing a formed steel part having a predominantly ferritic-bainitic structure.
2. Description of the Prior Art
In order to meet the demand in modern vehicle body construction for low weight combined with maximum strength and protection capacity, nowadays hot-press formed components, which are produced from high-strength steel, are used in such regions of the vehicle body, which in the event of a crash may be exposed to particularly high stresses. As examples of such formed steel parts A and B pillars, bumpers and door impact bars of automobile passenger vehicle are mentioned.
In hot-press hardening of steel blanks, which are slit from cold- or hot-rolled steel strip, the cut metal sheets concerned are heated to a deformation temperature usually above the austenitising temperature of the particular steel and placed in the heated state into the tool of a forming press. In the course of subsequent forming, the cut metal sheet or component formed thereof undergoes rapid cooling through contact with the cold tool, as a result of which hardened structure is produced in the component. In this case it may be sufficient if the component cools down without active cooling purely through contact with the tool. Fast cooling, however, can also be assisted if the tool itself is actively cooled down.
As reported in the article “Potentials for lightweight vehicle body construction”, appearing in the trade fair news-sheet of ThyssenKrupp Automotiv AG at the 61st International Motor Show 15-25 Sep. 2005, hot-press hardening is used in practice particularly for producing high-strength body components made of boron-alloyed steels. A typical example of such steel is the steel known under reference 22MnB5, which is to be found in the 2004 steel catalogue under material number 1.5528.
A steel comparable with steel 22MnB5 is known from JP 2006104526A. This known steel, apart from Fe and unavoidable impurities, contains (in % by weight) 0.05-0.55% C, max. 2% Si, 0.1-3% Mn, max. 0.1% P and max. 0.03% S. To increase the hardness, additionally amounts of 0.0002-0.005% B and 0.001-0.1% Ti can be added to the steel. In this case the particular Ti amount serves to bind the nitrogen contained in the steel. In this way the boron present in the steel can deploy its strength-enhancing effect to the maximum.
In accordance with JP 2006104526 A firstly sheets made of steel composed in this way are produced, which are then pre-heated to a temperature lying above the Ac3 temperature, typically in the range of 850-950° C. During subsequent rapid cooling from this temperature range in the pressing tool, the martensitic structure ensuring the desired high strengths is formed in the component press-formed from the respective cut metal sheet. In this case it is advantageous that the sheet metal parts heated to the temperature level mentioned can be transformed with relatively minimum deformation forces into complex shaped components. This is also valid in particular for such sheet metal parts as are produced from high-strength steel and provided with an anti-corrosive coating.
The components produced from boron-alloyed steels in the way described above reach strengths of over 1,500 MPa. However, as a consequence of the entirely martensitic structure of the components needed to do so, the components possess a residual elongation at break of 5-6%, which is not sufficient for many applications. The relatively low residual elongation at break is associated with low toughness. As regards applications, where good deformation behaviour is important in the event of a crash, this frequently leads to the situation where components produced from boron-alloyed steels in the known way no longer meet these requirements. This is the case in particular if the components being produced are parts for an automobile body.
In DE 10 2005 054 847 B3 it has been proposed, through subsequent heat treatment, to improve the crash behaviour of steel components produced by hot-press hardening which, apart from iron and unavoidable impurities, contain (in % by weight) 0.18-0.3% C, 0.1-0.7% Si, 1.0-2.50% Mn, max. 0.025% P, 0.1-0.8% Cr, 0.1-0.5% Mo, max. 0.01% S, 0.02-0.05% Ti, 0.002-0.005% B and 0.01-0.06% Al. In the course of the heat treatment, the hot-press hardened components are maintained at 320-400° C. Apart from the fact that such a heat treatment step can only be integrated at great expense in the established process chain for producing hot-press hardened steel components, practical trials have shown that the elongation at break of components heat-treated in this way worsens considerably.
Another possibility for producing a hardened metal component is known from DE 102 08 216 C1. With this known method a steel blank or pre-formed shaped component, which in each case consists of a steel of the type indicated above, is heated in a heating device to an austenitising temperature and then transported away to a hardening process. During the transport, sub-zones, of the first type, of the steel blank or shaped component, which should have higher ductility characteristics in the finished component, are quenched from a pre-determined cooling start temperature, lying above the γ-α-transformation temperature. This quenching is terminated when a given cooling stop temperature is reached, and to be precise before transformation to ferrite and/or pearlite or after only minimal transformation to ferrite and/or pearlite has taken place. Subsequently the steel blank or respective formed part is maintained in an isothermic manner for transforming the austenite into ferrite and/or pearlite. Meanwhile in the zones of the second type which, by comparison, should have lower ductility characteristics in the finished component, the hardening temperature is maintained just high enough that sufficient martensite formation can take place in the zones of the second type during a hardening process. Finally, cooling down then takes place. Additionally, the formed part obtained in a separate process step is dipped into a quenching tank or similar in order to produce the desired martensitic hardness structure. Also this operation requires a process step that can be integrated only at great expense into a modern production plant. Furthermore, components produced according to this known method also present the problem that, although they possess high strength, they are at the same time so brittle that they do not meet the demands for formability required in practice.